This invention relates to powder spray systems, and, more particularly, to a powder spray gun which can apply a wide pattern of particulate powder material upon a surface to be coated in a repeatable manner.
Particulate powder coating materials are commonly used to coat or paint objects in industrial finishing applications. In such applications, a particulate powder material such as epoxy, polyester or porcelain frit is conveyed to an applicator gun while entrained in an air stream and is sprayed from the nozzle of the gun onto a target surface or substrate. Conventionally the target substrate having powder loosely adhered thereto is then heated to melt the powder so that when the melted powder cools, it is permanently adhered to the substrate.
Powder spray guns used for spraying particulate powder materials generally include a barrel and a nozzle connected to one and of the barrel with a powder delivery passageway extending through the barrel. Particulate powder entrained in a stream of air flows through feed lines into the powder delivery passageway of the barrel from which it is sprayed through the nozzle onto the target surface to be coated.
The nozzles employed in prior art powder spray guns spray the particulate powder material along an axis generally parallel to the direction of flow of the powder through the powder delivery passageway in the gun barrel. The nozzle may be formed with a slot to produce a fan-shaped spray pattern, such as the Model 4A gun sold by the assignee of this invention, or, it may be provided with a deflector mounted exteriorly of the gun to deflect the stream of powder into a conical-shaped spray pattern as in U.S. Pat. No. 4,561,380.
One problem with prior art powder spray guns and nozzles is that it is difficult to obtain a repeatable, uniform spray pattern of particulate powder material upon the surface to be coated. This is particularly true in nonelectrostatic applications where an electrostatic field is not set up between the gun and workpiece to influence and encourage more uniform coating of the workpiece. It has been found that the particle density or concentration at one portion of the spray pattern may vary substantially from the particle density at another portion of the pattern. In addition, the particle density within the spray pattern may vary with time during a single spraying operation or run. It is believed that this uneven distribution of particulate powder material within the spray pattern is caused by the random and varying distribution of powder particles within the feed lines and powder delivery passageways upstream of the gun nozzle. Moreover, it has been observed that movement of the feed lines during or between spraying operations can cause a change in particle distribution within the spray pattern.
Another problem with prior art powder spray guns concerns the pattern typically produced by prior art powder spray nozzles having slotted discharge orifices. Generally these slotted nozzles spray a flat fan pattern which has a nominal rectangualar shape when it strikes the target surface. A closer inspection of the spray pattern produced by these prior art flat spray guns reveals, however, that "tails" or enlarged areas are produced at the ends of the spray pattern so that the overall pattern resembles a dog bone or barbell.
Another limitation experienced with prior art powder spray guns utilizing a slotted nozzle is that the effective width of the spray pattern is limited to about 18 inches. If a wider area must be covered, the spray patterns must be overlapped and this can result in streaking on the coated product caused by increased concentrations of the particulate powder material upon the surface to be coated at the point of pattern overlap.
A further disadvantage of prior art powder spray systems is that the velocity of the powder stream dispensed from the gun cannot be easily varied without varying the flow rate of the powder.